Black Holes

There are many popular myths concerning black holes,
many of them perpetuated by Hollywood. Television and movies have
portrayed them as time-traveling tunnels to another dimension, cosmic
vacuum cleaners sucking up everything in sight, and so on. It can be
said that black holes are really just the evolutionary end point of massivestars. But
somehow, this simple explanation makes them no less mysterious, and no
easier to understand.

Black holes: What are they?

Black holes are the evolutionary endpoints of stars at least 10 to 15
times as massive as the Sun. If a star that massive or larger undergoes
a supernova
explosion, it may leave behind a fairly massive burned-out stellar
remnant. With no outward forces to oppose gravitational forces, the remnant will collapse
in on itself. The star eventually collapses to the point of zero volume
and infinite density,
creating what is known as a "singularity."
Around the singularity is a region where the force of gravity
is so strong that not even light can escape. Thus, no information
can reach us from this region. It is therefore called a black hole, and
its
surface is called the "event
horizon."

But contrary to popular myth, a black hole is not a cosmic vacuum
cleaner. If our Sun was suddenly replaced with a black hole of the same mass,
Earth's orbit around the Sun would be unchanged. Of course,
Earth's temperature would change, and there would be no solar wind or solar
magnetic storms affecting us. To be "sucked" into a black hole, one has
to cross inside the
Schwarzschild radius. At this radius, the escape speed is equal to
the speed of
light, and once light passes through, even it cannot escape.

The Schwarzschild radius can be calculated using the equation for
escape speed:

vesc = (2GM/R)1/2

For photons, or objects with no mass, we can substitute c (the speed of light)
for Vesc and find the Schwarzschild radius, R, to be

R = 2GM/c2

If the Sun was replaced with a black hole that had the same mass as
the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's
radius of nearly 700,000 km). Hence the Earth would have to get very
close to get sucked into a black hole at the center of our Solar
System.

If we can't see them, how do we know they are there?

Since stellar black holes are small (only a few to a few tens of
kilometers in diameter), and light
that would allow us to see them cannot escape, a black hole floating
alone in space would be hard, if not impossible, to see in the visual
spectrum.

However, if a black hole passes through a cloud of interstellar matter, or is
close to another "normal" star, the black hole can accrete
matter into itself. As the matter falls or is pulled towards the black
hole, it gains kinetic energy, heats up and is squeezed by tidal
forces. The heating ionizes the
atoms, and when the atoms reach a few million Kelvin, they
emit X-rays. The
X-rays are sent off into space before the matter crosses the
Schwarzschild radius and crashes into the singularity.
Thus we can see this X-ray emission.

The optical companion of the black hole
candidate Cygnus X-1

Binary X-ray sources are also places to find strong black hole
candidates. A companion star is a perfect source of infalling material
for a black hole. A binary
system also allows the calculation of the black hole candidate's
mass.
Once the mass is found, it can be determined if the candidate is a neutron star
or a black hole, since neutron stars always have masses of about
1.5 times the mass of the Sun. Another sign of the presence of a black
hole is its random variation of emitted X-rays. The infalling matter
that emits X-rays does not fall into the black hole at a steady rate,
but rather more sporadically, which causes an observable variation in
X-ray intensity. Additionally, if the X-ray source is in a binary
system, and we see
it from certain angles, the X-rays will be periodically cut off as the
source is eclipsed by
the companion star. When looking for black hole candidates, all these
things are taken into account. Many X-ray satellites
have scanned the skies for X-ray sources that might be black hole
candidates.

Cygnus X-1 (Cyg X-1) is the longest known of the black hole
candidates. It is a highly
variable and irregular source, with X-ray emission that flickers in
hundredths of a second. An object cannot flicker faster than the time
required for light to travel across the object. In a hundredth of a second, light
travels 3,000 kilometers. This is one fourth of Earth's diameter. So
the region emitting the X-rays around Cyg X-1 is rather small. Its
companion star, HDE 226868 is a B0 supergiant with a surface
temperature of about 31,000 K. Spectroscopic
observations show that the spectral
lines of HDE 226868 oscillate with a period of 5.6 days. From the
mass-luminosity
relation, the mass of this supergiant is calculated as 30 times the
mass of the Sun. Cyg X-1 must have a mass of about 7 solar
masses, or it would not exert enough gravitational pull to cause
the wobble in the spectral lines of HDE 226868. Other estimate put the
mass of Cyg X-1 to as much as 16 solar masses. Since 7 solar masses
is too large to be a white dwarf
or neutron star, it must be a black hole.

An illustration
of Cygnus X-1, showing the companion star HDE 226868, the black
hole, material streaming from the companion to the black
hole, and the emission of X-rays near the black hole.

There are now about 20 X-ray binaries (as of early 2009) with known
black holes (from measurements of the black hole mass). The first of these,
an X-ray transient called A0620-00, was discovered in 1975, and the
mass of the compact object was determined in the mid-1980's to be
greater than 3.5 solar masses. This very clearly excludes a neutron
star, which has a mass near 1.5 solar masses, even allowing for all
known theoretical uncertainties. The best
case for a black hole is probably V404 Cygni, whose compact star is at
least 10 solar masses. There are an additional 20 X-ray binaries which
are likely to contain black holes - their behavior is the same as the
confirmed black holes, but mass measurements have not been possible.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.